Pyrolytic boron nitride (PBN) is formed by chemical vapor deposition of boron nitride in a reactor chamber by the vapor phase reaction of ammonia and a boron containing gas such as boron trichloride (BCl.sub.3) as is more specifically described in U.S. Pat. No. 3,152,006 which is incorporated herein by reference. The pyrolytic boron nitride is of very high purity and when separated or released from the substrate forms a self standing article of purified pyrolytic boron nitride.
A pyrolytic boron nitride heating unit includes a dielectric base of boron nitride and a heating element formed from a conductive material capable of resistive heating such as graphite and more particularly pyrolytic graphite. The heating element is connected to an external power supply to form a resistive heater. A pyrolytic boron nitride heating unit is used for resistive heating in a variety of system applications such as molecular beam epitaxy, space experiments, substrate heaters for electron microscopy and in the growth of superconducting films. In certain applications such as in the growth of superconducting films, it is necessary to introduce oxygen into the atmosphere of the reacting chamber in which the superconducting film is grown. The oxygen in the atmosphere will react with the graphite conductor in the heating unit to oxidize the conductor causing an open circuit. Existing electrical contacts for pyrolytic boron nitride heating units rely on a screw or clamp to press against the pyrolytic graphite conductor. This type of contact arrangement is not impermeable to a reactive gas and if the temperature at the point of contact with the graphite heating element is high enough such as 400.degree. C. oxidation will occur. In addition thermal stress can cause the screw or clamp to lose pressure at the point of contact which may cause arcing at the contact terminal and damage the heating unit.
Various methods have been attempted in the prior art to protect the electrical contact area from oxidation. One approach is to use a platinum coating to form a barrier between the pyrolytic graphite and the oxidizing atmosphere. In the extreme some users have operated the heating unit with a quartz envelop to protect the heating element from the oxidizing atmosphere. In other applications, the thermal stress of the installation can cause an arc at the point of electrical contact with the heating element which will damage the heating unit and render it nonfunctional.